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Mudstone on east beach of Lyme Regis, England East Beach 1 2006.JPG
Mudstone on east beach of Lyme Regis, England

Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from shale by its lack of fissility (parallel layering). [1] [2]


The term mudstone is also used to describe carbonate rocks (limestone or dolomite) that are composed predominantly of carbonate mud. [3] However, in most contexts, the term refers to siliciclastic mudstone, composed mostly of silicate minerals. [2]

The NASA Curiosity rover has found deposits of mudstone on Mars that contain organic substances such as kerogen. [4]


No one definition of a mudstone has gained general acceptance, [5] though there is wide agreement that mudstones are fine-grained sedimentary rocks, composed mostly of silicate grains with a grain size less than 0.063 millimetres (0.0025 in). [6] Individual grains this size are too small to be distinguished without a microscope, which means that most classifications emphasize texture rather than mineral composition, [5] and mudstones have historically received less attention from petrologists than have sandstones. [7] The simplest definition is that a mudstone is a fine-grained clastic sedimentary rock that is not laminated or fissile. [5] Most definitions also include a requirement that the rock contain significant amounts of both silt- and clay-sized grains. One common requirement is that a mudstone is a mudrock (a rock containing more than 50% silt- to clay-sized particles) in which between a third and two-thirds of the mud (silt and clay) fraction is clay particles. [7] [8] Another definition is that mudstone is a sedimentary rock in which neither silt, clay, nor coarser grains is predominant. [9] Rock of this composition that does show laminations or fissility is sometimes described as mudshale rather than mudstone. [8]

The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit. [1]

When the mineral composition of mudstones has been determined, using such techniques as scanning electron microscopy, electron probe microanalysis, or X-ray diffraction analysis, they have been found to be composed primarily of clay minerals, quartz, and feldspars, with a variety of accessory minerals. [10]

Carbonate mudstone

A Mudstone: few small components in a micritic matrix, width of picture is 32 mm DunhamMudstone.jpg
A Mudstone: few small components in a micritic matrix, width of picture is 32 mm

In the Dunham classification (Dunham, 1962 [11] ) system of limestones, a mudstone is defined as a mud-supported carbonate rock that contains less than 10% grains. Most recently, this definition has been clarified as a matrix-supported carbonate-dominated rock composed of more than 90% carbonate mud (<63 μm) component. [3]

The identification of carbonate mudstone

Thin section photomicrograph of carbonate mudstone 14 x2 PPL.jpg
Thin section photomicrograph of carbonate mudstone

A recent study by Lokier and Al Junaibi (2016) [3] has highlighted that the most common problems encountered when describing a mudstone is to incorrectly estimate the volume of 'grains' in the sample - in consequence, misidentifying mudstone as wackestone and vice versa. The original Dunham classification (1962) [11] defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan (1971 [12] ) to a grain size of less than or equal to 30 μm. Wright (1992 [13] ) proposed a further increase to the upper limit for the matrix size in order to bring it into line with the upper limit for silt (63 μm).

Mudstone mineralogy on Mars

Curiosity rover - mudstone mineralogy - 2013 to 2016 on Mars (CheMin; December 13, 2016)
NOTE: JK for "John Klein", CB for "Cumberland". CH for "Confidence Hills", MJ for "Mojave", TP for "Telegraph Peak", BK for "Buckskin", OD for "Oudam", MB for "Marimba", QL for "Quela", and SB for Sebina. (For locations/drillings, see image) PIA21146-MarsCuriosityRover-MudstoneMineralogy-20161213.png
Curiosity rover - mudstone mineralogy - 2013 to 2016 on Mars (CheMin; December 13, 2016)
NOTE: JK for "John Klein", CB for "Cumberland". CH for "Confidence Hills", MJ for "Mojave", TP for "Telegraph Peak", BK for "Buckskin", OD for "Oudam", MB for "Marimba", QL for "Quela", and SB for Sebina. (For locations/drillings, see image)

On December 13, 2016, NASA reported further evidence supporting habitability on the planet Mars as the Curiosity rover climbed higher, studying younger layers, on Mount Sharp. [15] Also reported, the very soluble element boron was detected for the first time on Mars. [15] In June 2018, NASA reported that Curiosity had detected kerogen and other complex organic compounds from mudstone rocks approximately 3.5 billion years old. [4] [16] [17] [18] [19] [20] [21] [22]

See also

Related Research Articles

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a type of carbonate sedimentary rock. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). A closely related rock is dolomite, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. In old USGS publications, dolomite was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones.

Shale A fine-grained, clastic sedimentary rock

Shale is a fine-grained, clastic sedimentary rock, composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock.

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation, transport, deposition and diagenesis. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures.

Siltstone Sedimentary rock which has a grain size in the silt range

Siltstone is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.

Conglomerate (geology) A coarse-grained clastic sedimentary rock with mainly rounded to subangular clasts

Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contain a matrix of finer grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.

Mudrock Class of fine grained siliciclastic sedimentary rocks

Mudrocks are a class of fine grained siliciclastic sedimentary rocks. The varying types of mudrocks include: siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 0.0625 mm and are too small to study readily in the field. At first sight the rock types look quite similar; however, there are important differences in composition and nomenclature. There has been a great deal of disagreement involving the classification of mudrocks. There are a few important hurdles to classification, including:

  1. Mudrocks are the least understood, and one of the most understudied sedimentary rocks to date
  2. It is difficult to study mudrock constituents, due to their diminutive size and susceptibility to weathering on outcrops
  3. And most importantly, there is more than one classification scheme accepted by scientists
Dunham classification

The Dunham classification system for carbonate sedimentary rocks was originally devised by Robert J. Dunham in 1962, and subsequently modified by Embry and Klovan in 1971 to include coarse-grained limestones and sediments that had been organically bound at the time of deposition. The modified Dunham Classification has subsequently become the most widely employed system for the classification of carbonate sedimentary rocks with 89% of workers currently adopting this system over the alternative Folk classification scheme

Clastic rock Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

Wackestone A mud-supported carbonate rock that contains greater than 10% grains

Under the Dunham classification system of limestones, a wackestone is defined as a mud-supported carbonate rock that contains greater than 10% grains. Most recently, this definition has been clarified as a carbonate-dominated rock in which the carbonate mud component supports a fabric comprising 10% or more very fine-sand grade or larger grains but where less than 10% of the rock is formed of grains larger than sand grade .

Gale (crater)

Gale is a crater, and probable dry lake, on Mars near the northwestern part of the Aeolis quadrangle at 5.4°S 137.8°E. It is 154 km (96 mi) in diameter and estimated to be about 3.5-3.8 billion years old. The crater was named after Walter Frederick Gale, an amateur astronomer from Sydney, Australia, who observed Mars in the late 19th century. Aeolis Mons is a mountain in the center of Gale and rises 5.5 km (18,000 ft) high. Aeolis Palus is the plain between the northern wall of Gale and the northern foothills of Aeolis Mons. Peace Vallis, a nearby outflow channel, 'flows' down from the hills to the Aeolis Palus below and seems to have been carved by flowing water. Several lines of evidence suggest that a lake existed inside Gale shortly after the formation of the crater.

Grainstone Type of limestone

Under the Dunham classification system of limestones, a grainstone is defined as a grain-supported carbonate rock that contains less than 1% mud-grade material. This definition has recently been clarified as a carbonate-dominated rock that does not contain any carbonate mud and where less than 10% of the components are larger than 2 mm. The spaces between grains may be empty (pores) or filled by cement.


Under the Dunham classification system of limestones, a packstone is defined as a grain-supported carbonate rock that contains 1% or more mud-grade fraction. This definition has been clarified by Lokier and Al Junaibi (2016) as a carbonate-dominated lithology containing carbonate mud in a fabric supported by a sand grade grain-size fraction and where less than 10% of the volume consists of grains >2 mm'.

Martian soil

Martian soil is the fine regolith found on the surface of Mars. Its properties can differ significantly from those of terrestrial soil, including its toxicity due to the presence of perchlorates. The term Martian soil typically refers to the finer fraction of regolith. So far, no samples have been returned to Earth, the goal of a Mars sample-return mission, but the soil has been studied remotely with the use of Mars rovers and Mars orbiters.

Aeolis quadrangle

The Aeolis quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Aeolis quadrangle is also referred to as MC-23 . The Aeolis quadrangle covers 180° to 225° W and 0° to 30° south on Mars, and contains parts of the regions Elysium Planitia and Terra Cimmeria. A small part of the Medusae Fossae Formation lies in this quadrangle.

Calcilutite is a type of limestone that is composed of predominantly, more than 50 percent, of either clay-size or both silt-size and clay-size detrital (transported) carbonate grains. These grains consist either of fossil fragments, ooids, intraclasts, pellets, other grains, or some combination of them. The term calcilutite was originally proposed in 1903 by Grabau as a part of his calcilutite, calcarenite and calcirudite classification system based upon the size of the detrital grains composing a limestone. In the original classification of limestone according to the dominant grain-size, calcisiltites were not named and are classified as calcilutite. In this classification, which the majority of geologists follow, a calcilutite consists of both silt- and clay-size, less than 0.062 mm in diameter, grains. It is the carbonate equivalent of a mudstone. Calcilutites can accumulate in a wide variety of marine and lacustrine environments.

Composition of Mars Branch of the Geology of Mars

The composition of Mars covers the branch of the geology of Mars that describes the make-up of the planet Mars.

Timeline of Mars Science Laboratory

The Mars Science Laboratory and its rover, Curiosity, were launched from Earth on November 26, 2011. As of November 13, 2020, Curiosity has been on the planet Mars for 2940 sols since landing on August 5, 2012. (See Current status.)

Yellowknife Bay, Mars

Yellowknife Bay is a geologic formation in Gale Crater on the planet Mars. NASA's Mars Science Laboratory Rover, named Curiosity, arrived at the low lying depression on December 17, 2012, 125 sols, or martian days, into its 668 sol planned mission on the planet. Primary mission goals of the Mars Science Laboratory were to assess the potential habitability of the planet and whether or not the Martian environment is, or has ever been, capable of supporting life. The site was chosen after much study of the region by previous missions. The Mars Reconnaissance Orbiter observed morphological features created by the presence of liquid water, suggesting the presence of an ancient lake which could have sustained microbial life. The geologic depression takes its name from the city Yellowknife, capital of the Canadian Northwest Territories, in honor of the 4 billion year old rock in the region surrounding the city, which matches the approximate age of the uncovered rock in Gale Crater.


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